The present invention relates generally to DC servo systems, and more particularly, to a cooperative differential drive system for use in material handling and flight control and guidance systems.
Many industrial handling systems, such as robot manipulators, numerically controlled machines, servo control systems, and other similar purpose force and position-oriented type devices, utilize a single DC servo motor as a primary drive source to provide mechanical locomotion. These types of electro-mechanical actuators demonstrate nonlinear behavior responses which can be attributed to several nonlinear dynamic effects when the actuator operates near stationary motion. The consequences of operating these single actuator drive systems near the region of their nonlinear disturbances is an overall performance degradation of both the position and force control of the system. In the course of completing a prescribed task, a material handling system often performs a number of operations that require the actuator to experience a transition through this region of nonlinear behavior. Consequently, material handling systems, that utilize the conventional single dedicated actuator drive system, have not been widely applied in complicated assembly tasks that require precise control and execution of both position and force trajectory paths.
Although the use of DC servo motors have become prolific in the robotics industry and other industrial devices, these electro-mechanical devices do however suffer from several restrictive performance disadvantages. Two of these disadvantages involve the following inherent dynamic nonlinearities when the actuator operates in the vicinity close to stationary locomotion.
The first disadvantage is know as the actuator dead zone phenomenon. Since DC motors are mechanical devices, they are often plagued by a variety of dissipative mechanical friction effects such as viscous friction and slip-stick friction. Slip-stick friction presents a particularly disturbing influence towards the actuator performance due to its discontinuous behavior often referred to as a "hard" nonlinearity. As a result, a sufficiently large input signal is needed to be applied before the stiction friction can be overcome and the motion of the actuator shaft can ensue. This phenomenon of a non-responsive output with the application of a significant input is commonly referred to as a dead zone.
The second disadvantage is known as the actuator start-up phenomenon. One of the advantages of using DC motors is that the actuator characteristics tend to be time invariant and linear within the normal operating range. By normal operation, we mean to infer that the actuator is operating in it's designed linear, time invariant region. However, in the initial start-up rotation phase of operation, the actuator system characteristics can be highly nonlinear and time-varying, which can be attributed to enrush currents, and other nonlinear physical phenomena.
These nonlinear dynamics pose a positional degradation to the operational performance of the system. For example, in the course of completing a prescribed task, a material handling system will often perform a variety of "stop and go", "up and down", and "forward and back" positional operations. These types of operations cause the system to experience a dead zone transition whenever the prescribed task requires the drive system to come to a complete stop as a consequence of performing a reversal in direction or when motion is resumed after a waiting period. In addition, when these operations are executed, the actuator is required to operate initially in the nonlinear actuator start-up region of operation. Ultimately, the presence of these nonlinearities results in the system being unable to accurately follow a prescribed position trajectory path. These nonlinear effects are typically further exasperated when the payload to the system is large, which is often the situation in many industrial applications.
Complicated control schemes have been suggested in an attempt to compensate for these undesirable effects. The literature is rich with suggested compensation control schemes. However, very limited success has been achieved in compensating for the so-called hard nonlinearities such as slip-stick friction. In general, these compensation schemes are very involved, computationally intense, and require an exact knowledge of the plant. In many instances, the resulting computed compensated control input signal becomes so unpractically large that neither the system power supplies are not able to achieve it, or the actuator does not have enough dynamic response to respond to the commanded signal. Also it is possible that the resultant system response could further be degraded by the excitation of other nonlinearities present in the system. Thus, for many applications, these nonlinear controller schemes are too impractical to be implemented.
In the past, the degrading effects of these nonlinearities on the performance of material handling systems were not of great concern since these systems tended to be classified as simple position oriented systems. In general, the tasks performed by these systems were of the simple "pick and place" category. These tasks did not require the system to achieve highly accurate end-point positioning, execute high precision trajectory path following routines, or maintain high resolution task repeatability levels. They generally only needed to meet some coarse, gross positioning requirements. Therefore, as a consequence of the poor position tolerance capability, these simple positioning systems could not dynamically interact with the environment. In the absence of environmental contact requirements, these systems were designed to meet the more important criteria of a stable operation while in the presence of nonlinear dynamics.
Recently, however, there has been considerable interest in utilizing material handling systems that perform a far wider range of flexible operations and have the ability to complete more sophisticated tasks. The envisioned type of tasks that will be performed require the system to execute highly accurate positioning trajectories, and also apply accurate forces to the environment. These types of tasks require the drive system to be much more than a simple positioning type of system, but rather requires it to dynamically perform environmental interactions. For example, these systems might be expected to perform tasks associated with the assembly of a complicated piece of hardware which requires the precise execution of position and force trajectory paths. Consequently, such future systems will dynamically control independently the position alone and the force applied to the environment. This issue of incorporating both position and force control is referred to in the literature as compliant motion control.
The issues and considerations involved in insuring a stable operation for force-controlled systems are often times entirely different than those posed for position-controlled systems, but rather adversely affects the dynamic behavior of the system. Over the years, investigations in the force control of robots have been conducted by a number of researchers. As previously stated, it has been shown that these nonlinear actuator dynamics does not adversely affect the stability of pure position-controlled systems. However, with the additional requirement of incorporating force control strategies in these systems, these aforementioned nonlinear actuator dynamics have become a problematic issue in resolving the system stability. It has been shown that as a consequence of the relatively high stiffness of the encountered environment, the force-controlled systems tend to exhibit highly underdamped behavior with much faster response characteristics compared to equivalent position-controlled systems. Consequently, under certain circumstances, the presence of these nonlinearities have induced surface chattering, limit cycles, and have exhibited other unstable behaviors in typical force-controlled systems. These aberrations are easily explained as the result of the actuator performing numerous "back and forth" operations in order to maintain the applied force. This actuator cogging action exacerbates the situation by causing the actuator to continually transverse the dead zone region. The actuator dead zone transition also introduces an actuator hysteresis force effect that limits the performance of the force-controlled system by not allowing the application of an accurate, repeatable force. As a result, even when a force-controlled system does operates in a stable mode, it is unable to accurately follow a desired force trajectory profile.
Thus it is an objective of the present invention to provide for a drive system that overcomes the limitations of conventional single actuator based drive systems. It is a further objective to provide an improved drive system for material handling and missile flight control systems, and the like, that require high resolution task repeatability capabilities and the execution of high precision position and force trajectory path following routines. It is a further objective to provide an improved drive system that may be used in complicated assembly tasks that require precise control and execution of both position and force trajectory paths.